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Evolution of the Magma Chamber beneath Usu Volcano since 1663: a Natural Laboratory for Observing Changing Phenocryst Compositions and Textures 总被引：2，自引：1，他引：2
We have investigated the evolution of an active silicic magma-feedingsystem beneath Usu volcano, Japan, where eight eruptions havebeen recorded since AD 1663. All magmatic products contain similartypes of plagioclase and orthopyroxene phenocrysts that consistof homogeneous cores with uniform compositions, and a zonedmantle that increases in size with time. The compositions ofplagioclase and orthopyroxene phenocrysts vary gradually andregularly with time, as do the bulk-rock compositions. The textureof these phenocrysts also changes systematically, caused byprogressive crystal growth, dissolution and diffusion. On thebasis of these observations, we conclude that the same magma-feedingsystem has persisted at Usu volcano since AD 1663. Compositionalvariation of magnetite phenocrysts differs from that of plagioclaseand orthopyroxene, because magnetite has large diffusion coefficientsand should represent magmatic conditions immediately beforethe eruption. Most pumices from Usu volcano contain two typesof magnetite phenocryst, each with a different composition andcrystallization temperature, indicating that two magmas mixedbefore each eruption (approximately several days before). Theend-members changed with time: rhyolite + basaltic andesite(1663); dacite ± rhyolite (1769, 1822, 1853); dacite± dacite (1977, 2000). The temperature of the magma apparentlyincreases with time, and the increase can be explained by sequentialtapping from a magma chamber with a thermal and chemical gradientin addition to injection of high-temperature magma. KEY WORDS: continuous existence of magma chamber; dacite; dissolution and diffusion of phenocrysts; magma mixing; magnetite 相似文献
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Petrographic and chemicoal studies of a suite of rocks fromthe Uwekahuna laccolith of Kilauea Volcano show that the originalmafic tholeiitic magma differentiated into tholeiitic picrite,tholeiitic olivine gabbro, and an aphanitic rock approachingquartz-basalt in composition. Mechanisms involved were an initialgravity settling of olivine and a final filter pressing of theresidual liquid. The range of composition represented by therocks of the laccolith is as great as that found among all hithertoanalysed lavas of the volcano.
2Present address: U.S. Geological Survey, Menlo Park, California 相似文献
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Reconstruction of an Evolving Magma Chamber beneath Usu Volcano since the 1663 Eruption 总被引：4，自引：3，他引：1
Usu volcano, located in northern Japan, has erupted seven timessince AD 1663. Before these seven eruptions, the volcano hada long repose period ( 5000 yr). The 1663 eruption was thefirstand by far the largest among the seven, producing nearlyaphyric rhyolitic pumice. Small mafic inclusions (‘micro-clots’J,consisting of glass, quenched crystals and abundant vesiclesoccur in the pumice. On the basis of petrological studies ofthe microclots, it is concluded that these are quenched meltsof a mafic magma injected into the rhyolite. The products ofthe 1769 eruption (and those of the following five eruptions)were dacites with abundant (10–15 vol %) microphenocrysts.According to crystal size distribution (CSD) analysis, the newmicrophenocrysts appear to have crystallized at a considerablyhigher cooling rate ( 300 times) than the phenocrysts in the1663 eruptive products. The contrasting petrologic featuresof the aphyric rhyolite and the following microphenocryst-richdacites can be explained by mixing and rapid cooling of a maficmagma injected during the 1663 eruption. We estimate the sizeof the magma chamber beneath Usu volcano just after the 1663eruption, using numerical calculations for a cooling magma chamber.If the magma chamber was sill-like, its thickness is estimatedto have been several hundreds of meters. KEY WORDS: Usu volcano; Japan; magma chamber evolution
*Corresponding author. Present address: Geomechanics, Earthquake Research Institute, The University of Tokyo, I-I-I Yayoi, Bunkyo-ku, Tokyo 113, Japan 相似文献
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Abstract In Japan low flat flood-prone areas are increasingly being developed. This paper describes a flood simulation model based on unsteady flow for calculating floods in such regions. The model is applied to the Tomoe River basin. 相似文献
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华北平原典型农田水、热与CO2通量的测定 总被引：42，自引：1，他引：42
在中国科学院栾城农业生态试验站用波文比-能量平衡法与涡度相关技术对净辐射通量（Rn)，潜热通量（λE），感热通量（H），土壤热通量（G）与冠层CO2通量（Fco2)进行了长期定位研究，结果显示Rn大部分用于作物潜热的消耗，连续2年4个生长季λE/Rn都在70%以上，在作物生育盛期，夏玉米λE/Rn略高于冬小麦，H/Rn都在15%左右，G/Rn在5%-13%之间，且冬小麦G/Rn明显高于夏玉米。蒸发比值（EF）在不同的理想环境条件下，随着可供能量（Rn-G)的增加表现出先迅速下降，后缓慢下降，最后趋于稳定的趋势，并在冬小麦环境条件下得到了验证，直角双曲线模型可以模拟Fco2随光通量密度（PPFD）的响应过程。晴天冠层水分利用效率（WUE）不是在正午出现最高值，当PPFD达到1500μmolm^-2s^-1左右时，WUE却略有下降。 相似文献
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